5.2 Modeling studies of fundamental ocean processes

From the various atmospheric GCM experiments described above, two
aspects of tropical ocean change in the 20th Century have been
implicated as forcing observed atmospheric change; i) the warming of
the warm pool region and ii) the nonstationarity of the ENSO time
series. Our research on fundamental ocean processes leads to the
hypothesis that these two oceanic changes are themselves coupled, and
in particular that the recent increase in El Niño amplitude is
consistent with the increase in warm pool temperature.

5.2.1 Decadal ENSO variability and the role of warm pool SST

From detailed analysis of the 1986-87 El Niño event, we find that El
Niño represents a mechanism by which the equatorial Pacific transports
heat poleward, a result subsequently confirmed from a more extended
study using NCEP data for the last 20 years. In particular, a
systematic relationship between the ocean heat content in the western
Pacific and the magnitude of El Niño warming was diagnosed for six
events since 1980-the higher the heat content in the western Pacific,
the stronger the subsequent El Niño warming (Fig. 5.7, top). The occurrence of warm pool heat
content maxima, at which time the zonal cross sections of Fig. 5.7 were made, precede the maxima in Niño 3
SST anomalies by 12-24 months.

Fig. 5.7 (a) Zonal distribution of upper ocean
heat content (0-260 m) in the equatorial belt (5S-5N) when the western
Pacific heat content reaches its pre-El Niño peak. Upper ocean heat
content used for this figure was smoothed in time using a Hanning
window with a width of 13 months. (b) The corresponding depth of the
20C isotherm. The ocean temperature used for calculating the 20C
isotherm depth was smoothed in time using a Hanning window with a
width of 13 months. (Based on results from Sun 2001,
J. Climate, submitted).

Our interpretation is that higher heat content in the western Pacific
is achieved through a deepening of the local thermocline, thus linking
the heat content in the western Pacific to the potential energy of the
ocean and thereby with the stability of the coupled ocean-atmosphere
system (Fig. 5.7, bottom). As warm-pool SST
initially increases, the zonal SST contrast also increases,
strengthening the trade winds. The stronger Walker circulation then
interacts with clouds and water vapor, allowing more solar radiation
to reach the ocean's surface over the east Pacific equatorial
cold-tongue, and at the same time, reducing the surface evaporative
cooling over that region. This is so because the impact of change in
the gradient term of the latent heat flux formula exceeds the impact
of increased wind speed. Through non-local ocean wave dynamics and
transports, heat content increases in the equatorial upper ocean of
the western Pacific warm pool. The resulting steeper tilt of the
equatorial thermocline is hypothesized to destabilize the coupled
system which is followed by energy release through a stronger El Niño.

To test this hypothesis, we constructed a coupled model. The
atmospheric model is statistical, with the equatorial surface winds
proportional to the zonal SST gradients. The ocean component is a
primitive equation model and therefore explicitly calculates the heat
budget of the entire equatorial upper ocean. The model produces
ENSO-like variations. The evolution of the subsurface ocean
temperature over the life cycle of the model El Niño resembles that of
observations (Fig. 5.8). In response to an
increase in warm pool SST, the model has a stronger El Niño. Similar
to the observational results, this simple model shows that an increase
in warm pool SST strengthens the zonal SST contrast during ENSO's cold
phase, which leads to an effective increase in the upper ocean heat
content in the warm pool. Stronger El Niño warming then follows, which
acts as a poleward heat pump. Of course, other processes can operate
to increase warm pool SSTs. In regard to the observed recent climate
change (see Fig. 5.1), it is
reasonable that the warm pool has increased due to local radiative
forcing related to the increase in anthropogenic gases. It is
hypothesized that this externally forced change may be influencing the
statistics of ENSO through the mechanisms described above.

5.2.2 North Pacific decadal ocean variability and the role of the tropics

As mentioned earlier, the time series of the ENSO index is correlated
with that of North Pacific SSTs (see Fig. 5.1), despite their different time
scales of variation. This reflects in part the well-known fact that
ENSO influences the North Pacific circulation, which in turn forces
North Pacific SSTs, on interannual time scales. The question we have
pursued is to what extent this atmospheric bridge between the tropics
and extratropics contributes to the decadal variability over the North
Pacific, including the Pacific Decadal Oscillation? We have addressed
this question by comparing the observed and simulated leading pattern
(EOF 1) and associated principal component time series (PC) of
wintertime North Pacific decadal SST variability (Fig. 5.9). The model results are obtained from the
ensemble average of 16 50-year GFDL R30 AGCM simulations in which
observed SSTs are specified in the tropical Pacific over the period
1950-1999 and a mixed layer model (MLM) is coupled to the AGCM
elsewhere over the global oceans. The EOFs are based on the monthly
SST anomalies that were first low-pass filtered to retain periods
greater than ~10 years and then the filtered values from November to
March were averaged together. The observed and MLM EOFs resemble each
other in several respects: they both explain about half of the
variance, and they are relatively well correlated in space and time,
with a spatial (temporal) correlation of 0.71 (0.69). The patterns in
Fig. 5.9 are very similar to those based on
unfiltered data which has conventionally been used to define the
PDO. The observed and simulated PCs are well correlated with the
filtered ENSO index time series, with correlation values of 0.77 and
0.90, respectively. In addition, maps of SST differences centered on
1976 (e.g., 1977-1988 minus 1970-1976, and 1977-1998 minus 1951-1976;
not shown) indicate that the "abrupt climate transition" in the model
and observations are similar and resemble the leading EOF but the
amplitude of the differences is approximately half as large in the
MLM. Overall, our model results suggest that a significant fraction of
the variance of the dominant pattern of low frequency SST variability
in the North Pacific is associated with the atmospheric bridge.

Fig. 5.9 EOF 1 of the low-pass filtered (>
~10 years) SST anomalies during November-March from (a) observations
and (b) the MLM. (c) The first principal component (time series
associated with EOF 1) of the low-pass SST anomalies from observations
(green line), the MLM (blue line) and low-pass ENSO index (black
line). The correlations (r) between the three time series are given
above (c). (Based on the results of Alexander, Blade, Newman, Lau, and
Lanzante, 2001, J. Climate, submitted).

Dynamic ocean processes likely play a fundamental role in climate
variability on decadal timescales. Rossby wave propagation can
introduce multi-year delays in the oceanic response to changes in the
atmospheric forcing. Subduction, where surface waters enter and flow
within the permanent pycnocline, provide a link between the
extratropical and tropical oceans over an ~8 year period. It has been
conjectured that when the subducted anomalies reach the equator, they
alter the equatorial SSTs and affect the North Pacific Ocean via the
atmospheric bridge, completing a circuit that enables decadal
oscillations. CDC has been involved in observational and modeling
studies that examine subduction and Rossby waves in the Pacific Ocean.

The standard deviation of the depth of the 25.5 isopycnal
surface, obtained from a global OGCM forced by observed surface
fluxes, indicates that there are three major centers of variability,
including: i) the Kuroshio region (30N, 160E), ii) along
the outcrop line at 35N between 180-140W, and iii) the tropics
between 10N-15N. The variability in the Kuroshio region reflects
changes in the ocean thermal structure resulting from the basin-wide
changes in the strength of the westerly winds that occurred in the
late 1970s and late 1980s. The thermocline changes lag changes in the
basin-wide wind stress curl forcing by 4-5 years, consistent with the
timescale of oceanic adjustment through Rossby wave propagation. The
second center of variability is associated with subduction, where it
has been proposed that thermal anomalies produced at the surface
primarily by anomalous heat fluxes, propagate equatorward along
isopycnals by the mean currents. Analyses of the OGCM and observations
has allowed us to track thermal anomalies from their source region
25N-35N, 140W-170W southwestward to ~18N over a period of ~8
years. South of this latitude thermocline variability appears to be
driven by local wind forcing.

The isopycnal depth changes in the subtropics of both hemispheres are
associated with large thermocline temperature variations in both the
OGCM and observations. We have examined the variability at 10N-15N by
comparing the OGCM results with those obtained from a simple Rossby
wave model forced by the same winds used in the OGCM simulation. The
evolution of thermocline depth is remarkably similar in the simple
model and OGCM (Fig. 5.10), indicating that a
substantial portion of the variability in the 10-15N latitude band
results from wind-forced baroclinic Rossby waves. Spectra and
Hövmoller diagrams based on low-pass filtered OGCM output indicate
that most of the thermocline variability occurs at periods longer than
~7 years. East of the dateline, subtropical Ekman pumping anomalies
exhibit variability over decadal periods and propagate westward at
speeds close to the phase speed of first baroclinic mode Rossby
waves. Thus, the spectral characteristics of the forcing may be
responsible for the enhanced oceanic response at low frequencies. The
thermocline signal that propagates across the basin at 13N, moves
southward along the western boundary and then eastward along the
equator (see Fig. 5.10). The low-frequency
variations of the thermocline depth along the equator may modulate the
amplitude and period of ENSO events on decadal timescales, an outcome
we plan to explore in the near future.

Fig. 5.10 (a) Evolution of the thermocline
depth along 13.6N (from east to west) as computed using a first-mode
baroclinic Rossby wave equation forced with the Ekman pumping derived
from the NCEP-NCAR reanalyses over the period 1958-1997. The equation
was solved using the method of characteristics. Contour interval is 10
m. Negative values (shallower thermocline) are shaded in blue, while
positive values (deeper thermocline) are shaded in red. (b) Same as in
(a), but computed from the NCAR OGCM. The 25.5 isopycnal has been used
as a proxy for thermocline depth. (c) Evolution of the depth of the
25.5 isopycnal along 130.8W from 13.6N to the equator. (d) Evolution
of the depth of the 25.5 isopycnal along the equator, from east to
west. In the OGCM anomalies originating along 13.6N can be tracked all
the way to the equator and along the equator. (Based on results of
Capotondi and Alexander, 2001, J. Phys. Oceanogr., in press).